How can I draw d -glucose in its chair conformation? Why it is the most common aldohexose in nature?

1 Answer
Jun 15, 2015

Answer:

First convert the Fischer projection to a Haworth projection, then convert the Haworth projection to a chair form.

Explanation:

The Fischer projection of glucose is

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Convert to a Haworth Projection

Step 1. Draw a basic Haworth projection with the ring oxygen at the top.

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#"C-1"# is the atom to the right of the oxygen, and #"C-5"# is the atom to its left.

Step 2. Draw a #"CH"_2"OH"# on #"C-5"#.

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Step 3. Draw an #"OH"# below the ring on #"C-1"# for the α form (draw it above the ring for the β form).

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Step 4. Draw all the #"OH"# groups on the right side of the Fischer projection on the bottom of the ring. Those on the left go above the ring.

The #"O"# on C-5 is part of the ring.

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You can omit the hydrogen atoms, so the Haworth projection for α-D-glucopyranose is

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Convert Haworth to Chair

Step 1. Draw a cyclohexane chair in which the #"O"# atom replaces #"C-6"# and the bulky #"CH"_2"OH"# is in the equatorial position.

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Step 2. Put all the #"OH"# groups that are "down" in the Haworth projection "down" in the chair. All the other #"OH"# groups go "up".

The chair form of α-D-glucopyranose is

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The structure of β-D-glucopyranose is

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Prevalence of Glucose

As you move around the β-glucose ring, you see that all the substituents are equatorial.

This is the most stable arrangement possible.

In α-glucose, only the #"OH"# at #"C-1"# is axial.

Every other aldohexose would have more axial substituents and be less stable.

Glucose is the most common hexose because it is the most stable.